---
title: "Static excitation system"
subject: "Power Plant Equipment"
chapter: 3
key: "semester_7/power_plant_equipment:static-excitation-system"
source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/static-excitation-system
site: BCT Engineering
---

# Static excitation system

Power Plant Equipment · Chapter 3: Var/Voltage Control in Hydrogenerating Systems

- Asked in: 2071 Chaitra, 5 marks
- Video: [Static Excitation System For 210MW Alternator in Power Plants [Hindi]](https://www.youtube.com/watch?v=D9RAyngvQ9s) — ElectricalTalks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/power_plant_equipment/static-excitation-system
- Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-3

## Question

Describe static excitation system with necessary diagrams.

## Short answer

A static excitation system uses thyristor rectifiers fed from the generator terminals through a transformer. The AVR adjusts the firing angle to control DC field current supplied via slip rings. It offers very fast response and high ceiling voltage, improving transient stability, but requires field flashing at start.

## Answer

A **static excitation system** has no rotating exciter. The field current is supplied by **thyristor (SCR) rectifiers** fed from the generator terminals through an excitation transformer, and is given to the rotor through slip rings and brushes.

```text
 Gen terminals --+--------------------------> to grid
                 |
            +----+----+
            | Excit.  |   step-down
            | transf. |
            +----+----+
                 | 3-ph AC
            +----+----+     firing   +-----+
            | Thyristor|<-----------| AVR |<-- PT, CT
            | bridge   |            +-----+
            +----+----+
                 | DC (field breaker)
              slip rings
                 |
            Rotor field winding
```

#### Working

1. A part of the generator output is stepped down by the excitation transformer.
2. A fully controlled 3-phase thyristor bridge rectifies it to DC.
3. The AVR compares the terminal voltage (from the PT) with the reference and changes the **firing angle** of the thyristors. A smaller firing angle gives a higher field voltage.
4. The DC is fed to the field through slip rings.
5. **Field flashing:** at start-up there is no terminal voltage (only a small residual voltage), so the field is first energised from the station battery or station supply until the generator voltage builds up.
6. A field breaker and a crowbar/discharge resistor de-excite the field on faults. The bridge can also invert (negative field voltage) for fast de-excitation.

#### Types
- **Potential-source (bus-fed)** static exciter: supply from a terminal transformer only.
- **Compound-source** static exciter: supply from both a PT and a series CT, so excitation is kept during a close-in fault.

#### Advantages
- Very fast response (time constant of a few ms) and high ceiling voltage; improves transient stability.
- No rotating exciter, so a shorter shaft and less maintenance.
- Fast field suppression by inverting the bridge.

#### Disadvantages
- Slip rings and brushes are still needed.
- The field supply depends on the terminal voltage; a close-in fault reduces excitation (unless compounded).
- Thyristors produce harmonics and need cooling.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. In a static excitation system, field current comes from:
   A. A shaft-mounted DC generator
   B. Thyristor rectifiers fed from generator terminals via an excitation transformer
   C. A battery only
   D. A diesel set
   Answer: B. There is no rotating exciter; thyristor bridges rectify stepped-down terminal voltage. (Key point: Source of field power)

2. How does the AVR control the field voltage?
   A. By changing the thyristor firing angle; a smaller angle gives higher field voltage
   B. By changing the firing angle; a larger angle gives higher field voltage
   C. By moving brushes
   D. By changing the transformer ratio
   Answer: A. A smaller firing angle increases the rectified DC output. (Key point: Control method)

3. Why is field flashing needed at start-up?
   A. There is no terminal voltage initially, so the field is first supplied from a battery or residual magnetism
   B. To cool the thyristors
   C. To test the breaker
   D. To increase frequency
   Answer: A. Since the source is the generator terminals, voltage must first be built up. (Key point: Field flashing)

4. Why is compound-source static excitation used?
   A. Using both PT and series CT keeps excitation during a close-in fault
   B. It removes slip rings
   C. It saves a transformer
   D. It slows response
   Answer: A. The CT supplies excitation when terminal voltage collapses in a close-in fault. (Key point: Compound source)

## Related in Power Plant Equipment

- [Excitation system with stabilizing transformer: transfer function](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aexcitation-stabilizing-transformer-model) — Power Plant Equipment, chapter 3, asked 13×
- [Dynamic response of excitation system](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aexcitation-dynamic-response) — Power Plant Equipment, chapter 3, asked 3×
- [Function of excitation system and its transfer function](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aexcitation-function-tf) — Power Plant Equipment, chapter 3, asked 2×
- [Brush excitation system](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Abrush-excitation-system) — Power Plant Equipment, chapter 3
- [Functional block diagram of excitation control](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aexcitation-control-block-diagram) — Power Plant Equipment, chapter 3
- [Need for excitation and improving dynamic response](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aexcitation-dynamic-improvement) — Power Plant Equipment, chapter 3

## Related in other subjects

- [Brushless versus static excitation](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Abrushless-vs-static-excitation) — Power Plant Design, chapter 4, asked 5×
- [Why recommend brushless over static excitation](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Abrushless-over-static-logic) — Power Plant Design, chapter 4
- [Static excitation pros and cons](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Astatic-excitation-pros-cons) — Power Plant Design, chapter 4
- [Working of brushless and static excitation](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Abrushless-static-working) — Power Plant Design, chapter 4